Electro-pneumatic control system

ABSTRACT

An electro-pneumatic braking system is disclosed for use on an air-braked vehicle. A conventional operator-operated brake control valve controls a relay valve which controls at least some of the brake actuators carried by the vehicle. An electronic control device compares the fluid pressure level at the delivery port of the brake control valve and at the brake actuators controlled by the relay valve. If the fluid pressure level at the delivery port of the brake control valve exceeds the pressure level at the brake actuators by more than a predetermined amount, a first error signal is generated which actuates a solenoid valve that causes the relay valve to initiate a brake application. If the pressure level at the brake actuators exceeds the pressure level at the delivery port of the brake control valve by more than a predetermined amount, a second error signal is generated which actuates a second solenoid valve that releases the relay valve, thereby also releasing the brakes of the vehicle.

United States Patent Morse et al.

[451 Mar. 12, 1974 ELECTRO-PNEUMATIC CONTROL SYSTEM Brian C. Deem, allof Elyria, Ohio [73] Assignee: Bendix-Westinghouse Automotive Air BrakeCompany, Elyria, Ohio 22 Filed: Apr. 16, 1971 21 Appl. No.: 134,572

[52] US. Cl 303/15, 303/20, 303/40 [51] Int. Cl B60t'13/68 [58] Field ofSearch 303/2, 3, 6, 7, l3, l5,

[56] References Cited UNITED STATES PATENTS 3,507,542 4/1970 Cannella303/3 Primary Examiner-Duane A. Reger i Attorney, Agent, or Firm--Ken C.Decker; William N.

Cooper et al. 303 20 5 7] ABSTRACT An electro-pneumatic braking systemis disclosed for use on an air-braked vehicle. A conventionaloperator-operated brake control valve controls a relay valve whichcontrols at least some of the brake actuators carried by the vehicle. Anelectronic control device compares the fluid pressure level at thedelivery port of the brake control valve and at the brake actuatorscontrolled by the relay valve. If the fluid pressure level at thedelivery port of the brake control valve exceeds the pressure level atthe brake actuators by more than a predetermined amount, a first errorsignal is generated which actuates a solenoid valve that causes therelay valve to initiate a brake application. If the pressure level atthe brake actuators exceeds the pressure level at the delivery port ofthe brake control valve by more than a predetermined amount, a seconderror signal is generated, which actuates a second solenoid valve thatreleases the relay valve, thereby also releasing the brakes of thevehicle.

18 Claims, 4 Drawing Eigures /2 ELECTRON/C co/vmoz.

TRA C 70/? 77A M E)? //V TE PFACE 1 V ELECTRO-PNEUMATIC CONTROL SYSTEMBACKGROUND OF THE INVENTION This invention relates to anelectro-pneumatic brak ing system for an air-braked vehicle.

Conventional air brake systems employ an operatoroperated brake controlvalve which actuates one or more relay valves that communicate theirassociated brake actuators with an air reservoir. Since pressureimpulses are propagated at the speed of sound in the control air linewhich extends between the brake control valve and the relay valve, therear brakes of the vehicle which are actuated by the relay valve areapplied after the front brakes are applied. While in shorter vehiclesthis time delay is so small that it is insignificant, in longervehicles, such as tractor-trailer combinations, the time delay may belong enough to cletrimentally affect braking performance. The problem isparticularly acute in so-called turnpike trains" which may consist oftwo or more tailers pulled by a single power unit. Therefore, it isdesirable to electrically propagate a control signal from the brakecontrol valve to the relay valve which actuates the brakes of thevehicle.

SUMMARY OF THE INVENTION Therefore, an important object of our inventionis to decrease the time delay between actuation of the brake controlvalve of an air-braked vehicle and application of the vehicles brakes.

Another important object of our invention is to electrically propagate'acontrol signal between the brake control valve and the relay valve.

A further object of our invention is to provide electronic controlcircuitry which actuates the relay valve of the system if the pressurelevel at the brake control valve differs from the pressure level at therelay valve by more than a predetermined amount.

A still further object of our invention is to provide fail-safecircuitry for detecting a malfunction in the control circuitry andactuating an appropriate warning device mounted in the operatorscompartment of the vehicle.

A still further object of our invention is to provide anelectro-pneumatic braking system which reverts to straight air operationwhen the electronic control portion of the system malfunctions.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic illustration of theair and electronic circuits of an electro-pneumatic braking system for acombination tractor-trailer vehicle made pursuant to the teachings ofour present invention;

FIG. 2 is a logic diagram of the electrical circuitry for the electroniccontrol unit used in the electropneumatic system illustrated in FIG. 1;

FIG. 3 is a'partial cross-sectional view of the relay valve used in thebraking system illustrated in FIG. 1; and v FIG. 4 is a partialcross-sectional view taken substantially along line 4-4 of FIG. 3.

DETAILED DESCRIPTION Referring now to FIG. 1 of the drawing, anelectropneumatic braking system for use on a tractor-trailer combinationvehicle is disclosed. The portion of the system normally carried by thetractor portion of the vehicle is indicated generally by the numeral 10and the portion of the system normally carried on the trailer portion ofthe vehicle is indicated generally by the nu meral 12. The interfacebetween the tractor and the trailer portions of the system is indicatedgenerally by the dashed line 14. The system includes an electroniccontrol unit indicated generally by the numeral 16 which may be carriedon either the tractor or the trailer portion of the vehicle; however,the electronic control unit 16 is illustrated in FIG. 1 as being carriedby the trailer.

The portion of the braking system carried by the tractor includes astandard air compressor 18 which is powered by the engine of thevehicle. The air compressor 18 compresses atmospheric air to charge airreservoir 20. The reservoir 20 is communicated to the inlet of astandard operator-operated brake control valve 22 by a conduit 24. Theconduit 24 also communicates the reservoir 20 with the supply port of astandard air relay valve 26 of a type well known to those skilled in theart. When a brake application is effect-ed, the relay valve 26communicates air from the reservoir 20 to the rear brakes of the tractorthrough conduits 28. The outlets of the brake control valve 22 arecommunicated to the front brakes of the tractor through conduit 30, tothe service port of the relay valve 26 through conduit 32 and to theservice port of a second relay valve 34 carried by the trailer throughconduits 36 and 38 which are joined together at the tractor-trailerinterface by an appropriate coupling 40. The compressed air is alsocommunicated from the reservoir 20 to a second reservoir 42 carried bythe trailer through a conduit 44 which is coupled to a branch of theconduit24 at the tractor-trailer interface by an appropriate connector46. The delivery section of the relay valve 34 is communicated to thetrailer brakes by conduits 48 and 48a. As is well known to those skilledin the art, when the vehicle operator effects a brake application uponoperation of the brake valve 22, pressurized air from the reservoir 20is communicated to the front brake actuators through the conduit 30, tothe service port of the relay valve 26 through conduit 32, and to theservice port of the relay valve 34 through the conduits 36 and 38. Relayvalves 26 and 34 are thereby operated to communicate pressurized airfrom the reservoirs 20 and 42 to the tractor and trailer brakeactuators.

A pressure transducer 50 is communicated to the brake valve 22 andproduces an output which is proportional to the pressure level developedat the brake control valve 22 when a brake application is effected. Theoutput of the pressure transducer 50 is transmitted to the electroniccontrol unit .16 through leads 52 and 54 which are joined together atthe tractor-trailer interface by an appropriate connector 56. A secondpressure transducer 58, which may be identical to the transducer 50,generates an output proportional to the pressure level developed at thedelivery ports of the relay valve 34. The output of the transducer 58 istransmitted to the electronic control unit 16 through the lead 60. Thetransducers 50 and 58 may be of the piezo-resistive type. A lead 62transmits the output signal of the eleca manner more completelydescribed hereinafter. A

lead 68 connects a second output of the control unit 16 with a secondsolenoid valve 70 carried by the relay valve 34 which controls fluidcommunication between the primary chamber of the latter and theatmosphere.

Referring now to FIGS. 3 and 4, the relay valve 34 includes a housing 72defining a chamber 74 therewithin. The housing 72 is provided with asupply port 75 which is communicated with the conduit 44. The port 75 iscommunicated to an intermediate chamber 76 which in turn is communicatedto the reservoir 42. A flange 78 on the housing 72 secures the relayvalve 34 to a suitable mounting boss provided on the reservoir 42.

A piston 80 is slidably mounted in the chamber 74 and divides the latterinto a delivery section 82 and a primary section 84. An annular valvemember 86 defines a passage 88 therewithin which normally communicatesthe delivery chamber 82 with the atmosphere through an exhaust port 90.A Spring 92 yieldably urges the valve member 86 into sealing engagementwith the annular valve seating area 94 provided on the housing 72 tothereby prevent fluid communication between the intermediate chamber 76and the delivery chamber 82. The piston 80 includes a valve operatingportion 96 which is adapted to operate the valve member 86. A spring 98yieldably urges the piston 80 and therefore the valve operating portion96 away from the valve member 86. As is well known to those skilled inthe art, delivery ports (not shown) communicate the delivery chamber 82with the conduits 48 and 48a, which communicate relay valve 34 with thetrailer brake actuators.

Relay valve 34 is further provided with a service port 100, which iscommunicated to the brake valve 22 by the conduits 3 6 and 38, and asecond supply port 102 which is communicated to the reservoir 42 by theconduit 66. Each of the ports 100 and 102 is communicated with a valvecompartment generally indicated by the numeral 104 within the housing72. Valve seats 106 and 108 divide the compartment 104 into a centralsection 110 and a pair of end sections 112 and 114. As is illustrated inFIG. 4, the service port 100 is communicated with the end section 112and the supply port 102 is communicated with the end section 114. Apassage 116 communicates the central section 110 with the primarysection 84 of the chamber 74. A valve member 118 is slidably mounted inthe central section 110 and is provided with a valve operating stem 120which extends through the valve seat 106 to engage the end of anarmature 122 which forms a part of the solenoid valve 64. A spring 124yieldab ly urges the armature 122 to the left viewing FIG. 4, andanother spring 126 yieldably urges the valve member 118 toward the valveseat 106. However, since the spring 124 is stronger than the spring 126,the valve member 118 is normally maintained in sealing engagement withthe valve seat 108 and flow of air is permitted through the valve seat106. The solenoid valve 64 further includes a pole piece 128 and anelectrical coil 130. As is well known to those skilled in the art, whena current is passed through the coil 130, the armature 122 will be movedto the right viewing FIG. 4, against the bias of the spring 124, therebypermitting the spring 126 to urge the valve member 118 away from thevalve seat 108 and into sealing engagement with the valve seat 106.

Relay valve 34 further includes a second exhaust port 132 which iscommunicated to a valve chamber 134 by a conduit 136. The chamber 134 isalso communicated with the primary chamber 84 by a passage 138. A springyieldably urges an armature 142 which is a part of the solenoid valve70, to the left viewing FIG. 4. The armature 142 carries a valve member144 which is urged into sealing engagement with valve seat 146 by thespring 140 to normally prevent fluid communication between the conduit136 and the valve chamber 134. The solenoid valve 70 further includes apole piece 148 and coil 150. As is well known to those skilled in theart, when an electrical current is passed through the coil 150, thearmature 142 is urged to the right viewing FIG. 4, thereby permittingfluid communication from the chamber 134 into the conduit 136 andthrough second exhaust port 132 to atmosphere Referring now to FIG. 2,the circuitry included within the electronic control unit 16 will now bedescribed in detail. The input T; from the transducer 50 is fed into aninverter 152 and is also fed into an adder 154. Similarly, the input Tfrom the transducer 58 is fed into inverter 156 and also into an adder158. The output of the inverters 152 and 156 are fed into the input ofthe adders 158 and 154, respectively, so that the output of the adder154 is equal to the quantity T minus T and the output of the adder 158is equal to the quantity T minus T The output of the adder 154 is fedinto the input of a comparator 160 which compares the quantity T minus Tto some referencevalue. The reference voltage A is ideallyrepresentative of a 5 psi pressure level. Therefore, the pressure at thebrake control valve 22 must exceed the pressure in the conduits 48 and48a by at least 5 psi before the trailer brakes are actuated by theelectronic control. Similarly, the output T minus T is compared to areference level in a comparator 162. The reference level used in thecomparator 162 is ideally also approximately 5 psi. The output of thecomparator 160 triggers a 250 millisecond one-shot 164, the output ofwhich is connected to one of the inputs of an AND gate 166. The outputof the comparator 160 is also fed to the AND gate 166. The third inputof the AND gate 166 is connected to the output of a window comparator168 which compares the input T, with two reference levels B and C. Thereference level B is proportional to approximately a 3 psi pressurelevel to assure that the vehicle operator has made the brakeapplication. The reference level C is proportional to a pressure levelof approximately 150 psi to assure that a malfunction in the controlsystem has not resulted in a completely unreasonable input T The outputof the AND gate 166 activates a power amplifier 170 which drives thesolenoid valve 64. Since the output of the AND gate 166 terminates afterthe one-shot 164 times out, the solenoid valve 64 can be actuated for amaximum of 250 milliseconds. This is a sufficiently long time period toassure that the pressure in the primary chamber 84 will become equal tothe pressure level at the brake valve 22, but prevents a malfunction inthe electronic control unit from actuating the brakes of the trailer foran indefinite period. Similarly, the output of the comparator 162 isconnected to the input of a 500 millisecond one-shot 172, the output ofwhich is connected to an input of AND gate 174. The output of thecomparator 162 is also connected directly to the AND gate 174. Theoutput of the AND gate 174 is connected to a power amplifier 176 whichdrives the solenoid valve 70 which exhausts the relay valve 34, therebyrelieving the trailers brakes. The AND gate 174 produces an output onlyuntil the one-shot 172 times out so that the relay valve 34 is exhaustedfor a maximum of 500 milliseconds thereby preventing a malfunction inthe electronic control unit 16 from exhausting the trailers brakes foran indefinite time period.

To indicate a failure in the electronic control unit 16 or in thetransducers 50 and 58, exclusive OR gates 178 and 180 are used. Oneinput of the exclusive OR gate 178 is connected to the output of thecomparator 160 and another input of the exclusive OR gate 178 isconnected to the output of AND gate 166. The output of exclusive OR gate178 is connected to an input of an OR gate 182, the output of whichactuates a fail-safe light in the operators compartment of the vehicle.If the output of the comparator 160 is different from the output of theAND gate 166, a failure exists and the exclusive OR gate 178 and the ORgate 182 actuate the fail-safe light. Similarly, one input of theexclusive OR gate 180 is connected to the. output of the comparator 162and the other input of the exclusive OR gate 180 is connected tothe-output of AND gate 174. If the output of the comparator 162 isdifferent from the output of the AND gate 174, a failure exists and theexclusive OR gate 180 and the OR gate 182 turn on the fail-safe light.

Exclusive NOR gates 184 and 185 indicate a failure in the powertransistors 186a, 186b, or 186a which are included in the poweramplifier 170 and the transistors- 188a, 188b, and 1886 which areincluded within the power amplifier 176, respectively. One of the inputsof the exclusive NOR gate 184 is connected to the base of the transistor186a and the other input of the exclusive NOR gate 184 is connected tothe collector of the transistor 186a. If the'two inputs to theexclusiveNOR gate 184 are the same, a failure exists and the exclusiveNOR gate 184 and the OR gate 182 turn on the fail-safe light. Similarly,one of the inputs to the exclusive NOR gate 185 is connected to the baseof the transistor 188a and the other input of the exclusive NOR gate 185is connected to the collector of the transistor 188a. If the two inputsof the exclusive NOR gate 185 are the same, a failure exists and theexclusive NOR gate 185 cooperates with the OR gate 182 to turn on thewarning device in the vehicle operatorscompartment.

MODE OF OPERATION brake valve 22 is sensed by the transducer 50 and istransmitted to the control unit 16. When the pressure level at the brakevalve 22 is increased to obtain a level of approximately 5 psi above thelevel of the output of the relay valve 34, the comparator 160 producesan output signal which is fed through the AND gate 166 which in turnactuates the power amplifier 170 to actuate the solenoid valve 64. Whenthis occurs, the armature 122 of the solenoid valve 64 is urged to theright viewing FIG. 4, thereby permitting the spring 126 to urge thevalve member into sealing engagement with the valve seat 106, therebyterminating fluid communication between the service port 100 and theprimary chamber 84. Simultaneously, the other end of the valve member118 moves away from the valve seat 108 per-- mitting air to flow fromthe reservoir 42 into the primary section 84 through the passage 116.Air in the primary section 84 forces the piston downwardly against thebias of the spring 98.

As the piston 80 moves, the valve operating member 96 first engages thevalve member 86 to terminate the communication between the deliverysection 82 and the exhaust port 90. Thereafter, further movement of thepiston 80 urges the valve member 86 away from the valve seat 94 topermit high pressure air in the reservoir 42 to be communicated into thedelivery section 82. As mentioned hereinabove, the high pressure air inthe delivery section 82 is communicated to the trailer brake actuatorsthrough delivery ports (not shown) in the relay valve 34 and through theconduits 48 and 48a. When sufficient air pressure is communicated to thetrailer brake actuators such that the transducer 58 senses a pressurewhich is approximately 5 psi below that being sensed by transducer 50,the output of the comparator terminates, thereby turning off the poweramplifier to thereby release the solenoid valve 64, to permit the valvemember 118 to be returned to the position illustrated in FIG. 4.Thereafter, air communicates through the conduits 36 and 38 until normalbraking pressure balance is achieved.

When the vahicle operator releases the brake control valve 22, such thatthe pressure transducer 50 senses apressure in the brake control valve22 which is approximately 5 psi below the pressure sensed by thetransducer 58, the comparator 162 and! AND gate 174 produce an outputwhich actuates the power amplifier 176. Power amplifier 176 drives thesolenoid valve 170 which urges the armature 142 carrying valve 144 awayi from the valve seat 146 to permit the primary section 84 to exhaustthrough the exhaust port 132 through the passage 138, chamber 134, andconduit 136. When this occurs, the spring 98 urges the piston 80 towardthe position illustrated in FIG. 3, thereby reclosing valve member 86against the valve seat 94 to prevent communication of supply air intothe delivery chamber 82 and to permit the delivery chamber 82 to exhausttoatmosphere through the exhaust port 90. Since the brake actuators arecommunicated to the delivery chamber 82, the brakes of the vehicle willthereby'be released.

It should be noted that the relay valve 34 will operate I as a standardrelay valve when the pressure levels between the brake valve 22 andrelay valve 34 are within predetermined limits or when the electroniccontrol.

unit malfunctions, since service air is supplied to the service port loofrom the brake valve 22 through the conduits 36 and 38. As describedabove, when the solenoid valve 64 is released, the valve member 118 isurged away from the valve seat 106 and fluid communielectronic controlunit and the components operate at substantially the speed of light.Therefore, the time delay between an application of the brake valve 22by the vahicle operator and application of the trailer brakes will bemuch less when the electrical control unit is working normally than whenthe malfunction re,-

quires that the relay valve be operated by service air from the brakecontrol valve 22.

We claim:

1. In a vehicle brake actuation system having a fluid pressure sourceand brake actuators operated by fluid pressure:

an operator-operated brake control valve communicated with said fluidpressure source;

first means for generating a first signal proportional to the differencebetween the pressure levels at said brake control valve and the pressurelevel delivered to said brake actuators;

second means for comparing said first signal with a predeterminedreference signal and generating a second signal if said first signal isgreater than the reference signal and terminating said second signal ifthe first signal again drops below said reference signal; and I thirdmeans responsive to said second signal for initiating'communicationbetween said brake actuators and said pressure source if the pressurelevel delivered to the brake actuators is less than the pressure levelat the brake control valve and venting said brake actuators if thepressure level in the brake actuators is greater than the pressure levelat said brake control valve.

2. In a vehicle brake actuation system having a fluid pressure sourceand brake actuators operated by fluid pressure;

an operator-operated brake control valve communicated with said fluidpressure source;

control means for comparing the fluid pressure level at the brakecontrol valve and the pressure level delivered to the brake actuatorsand generating a first error signal if the pressure level at the brakecontrol valve exceeds the pressure level delivered to said actuators bymore than a predetermined amount and terminating said first error signalif the pressure differential of the brake control valve over the brakeactuators decreases to a value less than the predetermined amount, saidcontrol means including means for generating a second error signal ifthe pressure level delivered to said brake actuators exceeds thepressure level at said brake control valve by more than a predeterminedamount and terminating said second error signal if the pressuredifferential of the brake actuators over that of the brake control valvedecreases to a value less than the predetermined amount; and

valve means responsive to said first error signal to communicate saidbrake actuators with said fluid pressure source and responsive to saidsecond error signal to vent said brake actuators.

3. The invention of claim 2:

said control means including first means for generating a firstintermediate signal proportional to the difference between the pressurelevel at the brake control valve and the pressure level delivered to thebrake actuators and second means for generating a second intermediatesignal proportional to the difference between the pressure leveldelivered to said brake actuators and the pressure level at said brakecontrol valve.

4. Theinvention of claim 3:

said control means further including a first comparator for comparingsaid first intermediate signal with a first reference signal andgenerating said first- 8 error signal if the first intermediate signalexceeds the first reference signal and a second comparator for comparingsaid second intermediate signal with a second reference signal andgenerating said second error signal if the second intermediate signal isgreater than the second reference signal.

5. The invention of claim 3:

said control means including first pressure responsive means at saidbrake valve for generating an electrical signal proportional to thepressure level at said brake valve; and

second pressure responsive means for generating an electrical signalproportional to the pressure level delivered to said brake actuators;

said electrical signal generated by said first and second pressureresponsive means being transmitted to said first means and to saidsecond means.

6. The invention of claim 5:

said first and second pressure responsive means being piezo-resistivepressure transducers.

7. The invention of claim 2; and r a relay valve for controlling fluidcommunication between the pressure source and the brake actuators;

said brake control valve being operable to transmit pressure to saidrelay valve to actuate the latter to initiate fluid communicationbetween the brake actuators to the pressure source when the brakecontrol valve is applied and to release said relay valve to vent saidbrake actuators when the brake control valve is released;

said valve means responding to said first error signal to actuate therelay valve to thereby initiate communication between said brakeactuators and the pressure source and to said second error signal torelease said relay valve to vent said brake actuators.

8. The invention of claim 7:

said valve means including a first electrically operated valveresponsive to said first error signal to terminate fluid communicationbetween the relay valve and the brake control valve and to initiatefluid communication between the pressure source and the relay valve toactuate the latter to admit fluid pressure to the brake actuators and asecond electrically operated valve responsive to said second errorsignal to release said relay valve to vent said brake actuators.

9. The invention of claim 2:

a warning device mounted in the operators compartment of the vehicle;and

fail-safe means for actuating said warning device when a malfunctionprevents satisfactory operation of said control means.

10. The invention of claim 4:

said control means further including a device responsive to the outputof one of said comparators for generating a signal for a predeterminedtime period, the length of said time period being independent offluctuations in the value of said error signals and first gating meansinhibiting the output of said one comparator after the signal generatedby said device expires.

11. The invention of claim 10:

said control means further including a device responsive to the outputof the other comparator for generating a signal for a predetermined timeperiod,

the length of said time period being independent of fluctuations in thevalue of said-error signals, and second gating means inhibiting theoutput of the other comparator after the signal generated by said deviceexpires.

12. The inventionof claim 4:

said control means further including a device responsive to the outputof said first comparator for generating a signal for a predeterminedtimeperiod, a window comparator for comparing said first intermediate signalwith a pair of reference signals and generating an output signal if thefirst intermediate signal is greater than one of the reference signalsbut less than the other reference signal, and gating means inhibitingthe output of said first comparator unless said window comparator andsaid device produces an outputsignal.

13. The invention of claim 10; and

a warning device mounted in the operators compartment of the vehicle;

said control means further including means for actuating said warningdevice if the output of the corresponding gating means is not equal tothe output of said one comparator.

14. The invention of claim 11; and

a warning device mounted in the operators compartment of the vehicle;

said control means further including means for actuating said warningdevice if the output of said first gating means is not equal to theoutput of said one comparator and if the output of said second gatingmeans is not equal to the output of the other comparator.

15. The invention of Claim 4:

a warning device mounted in the operators compartment of the vehicle;and n first and second power amplifiers for amplifying said first andsecond error signals respectively;

said control means further including means for 'actuating said warningdevice when either of said first or second power amplifiers malfunction.16. In a tractor-trailer breaking system having a fluid pressuresourceand brake actuators operated by fluid pressure:

an operator-operated brake control valve communicated to-s'aid firstreservoir for-actuating the brakes of the tractor and trailer;

control means for comparing the fluid pressure level at the brakecontrol valve and the fluid pressure 5 level at the brake actuatorscarried by the trailer and generating a first error signal if thepressure level at the brake control valve exceeds the pressure leveldelivered to said trailer actuators by more than a predetermined amountand generating a second error signal if the pressure level delivered tosaid trailer brake actuators exceeds the pressure level at said controlvalve by more than a predetermined amount;

said electrically operated valve :means being responsive to said firsterror signal to communicate said primary section of the relay valve withsaid second reservoir to thereby cause said delivery section tocommunicate said second reservoir with the brake actuators carried bythe trailer and responsive to said second error signal to vent saidprimary section to atmosphere to thereby permit said delivery section toalso vent said trailer brake actuators to atmosphere.

17. The invention of claim 16:.

said brake control valve being communicated to said primary section toactuate said relay valve when a malfunction prevents-normal operation ofthe electrically operated valve means;

said electrically operated valve means terminating fluid communicationbetween the brake valve and the primary section when communicationbetween said second reservoir and said primary section is initiated. t i

5 3 pressure source and brake actuators operated by fluid pressure:

an operator-operated brake control valve communicated with said fluidpressure source; control means for comparing the difference between saidfluid pressure level at the brake control valve and at said actuatorswith a first reference value and generating a first error signal if saiddifference exceeds said reference value and terminating said first errorsignal when said difference drops below said predetermined amount, saidcontrol means comparing the difference between the fluid presnal to ventsaid actuators.

18. In a vehicle brake actuation system having a fluid

1. In a vehicle brake actuation system having a fluid pressure sourceand brake actuators operated by fluid pressure: an operator-operatedbrake control valve communicated with said fluid pressure source; firstmeans for generating a first signal proportional to the differencebetween the pressure levels at said brake control valve and the pressurelevel delivered to said brake actuators; second means for comparing saidfirst signal with a predetermined reference signal and generating asecond signal if said first signal is greater than the reference signaland terminating said second signal if the first signal again drops belowsaid reference signal; and third means responsive to said second signalfor initiating communication between said brake actuators and saidpressure source if the pressure level delivered to the brake actuatorsis less than the pressure level at the brake control valve and ventingsaid brake actuators if the pressure level in the brake actuators isgreater than the pressure level at said brake control valve.
 2. In avehicle brake actuation system having a fluid pressure source and brakeactuators operated by fluid pressure; an operator-operated brake controlvalve communicated with said fluid pressure source; control means forcomparing the fluid pressure level at the brake control valve and thepressure level delivered to the brake actuators and generating a firsterror signal if the pressure leVel at the brake control valve exceedsthe pressure level delivered to said actuators by more than apredetermined amount and terminating said first error signal if thepressure differential of the brake control valve over the brakeactuators decreases to a value less than the predetermined amount, saidcontrol means including means for generating a second error signal ifthe pressure level delivered to said brake actuators exceeds thepressure level at said brake control valve by more than a predeterminedamount and terminating said second error signal if the pressuredifferential of the brake actuators over that of the brake control valvedecreases to a value less than the predetermined amount; and valve meansresponsive to said first error signal to communicate said brakeactuators with said fluid pressure source and responsive to said seconderror signal to vent said brake actuators.
 3. The invention of claim 2:said control means including first means for generating a firstintermediate signal proportional to the difference between the pressurelevel at the brake control valve and the pressure level delivered to thebrake actuators and second means for generating a second intermediatesignal proportional to the difference between the pressure leveldelivered to said brake actuators and the pressure level at said brakecontrol valve.
 4. The invention of claim 3: said control means furtherincluding a first comparator for comparing said first intermediatesignal with a first reference signal and generating said first errorsignal if the first intermediate signal exceeds the first referencesignal and a second comparator for comparing said second intermediatesignal with a second reference signal and generating said second errorsignal if the second intermediate signal is greater than the secondreference signal.
 5. The invention of claim 3: said control meansincluding first pressure responsive means at said brake valve forgenerating an electrical signal proportional to the pressure level atsaid brake valve; and second pressure responsive means for generating anelectrical signal proportional to the pressure level delivered to saidbrake actuators; said electrical signal generated by said first andsecond pressure responsive means being transmitted to said first meansand to said second means.
 6. The invention of claim 5: said first andsecond pressure responsive means being piezo-resistive pressuretransducers.
 7. The invention of claim 2; and a relay valve forcontrolling fluid communication between the pressure source and thebrake actuators; said brake control valve being operable to transmitpressure to said relay valve to actuate the latter to initiate fluidcommunication between the brake actuators to the pressure source whenthe brake control valve is applied and to release said relay valve tovent said brake actuators when the brake control valve is released; saidvalve means responding to said first error signal to actuate the relayvalve to thereby initiate communication between said brake actuators andthe pressure source and to said second error signal to release saidrelay valve to vent said brake actuators.
 8. The invention of claim 7:said valve means including a first electrically operated valveresponsive to said first error signal to terminate fluid communicationbetween the relay valve and the brake control valve and to initiatefluid communication between the pressure source and the relay valve toactuate the latter to admit fluid pressure to the brake actuators and asecond electrically operated valve responsive to said second errorsignal to release said relay valve to vent said brake actuators.
 9. Theinvention of claim 2: a warning device mounted in the operator''scompartment of the vehicle; and fail-safe means for actuating saidwarning device when a malfunction prevents satisfactory operation ofsaid control means.
 10. The invention of claim 4: said controL meansfurther including a device responsive to the output of one of saidcomparators for generating a signal for a predetermined time period, thelength of said time period being independent of fluctuations in thevalue of said error signals and first gating means inhibiting the outputof said one comparator after the signal generated by said deviceexpires.
 11. The invention of claim 10: said control means furtherincluding a device responsive to the output of the other comparator forgenerating a signal for a predetermined time period, the length of saidtime period being independent of fluctuations in the value of said errorsignals, and second gating means inhibiting the output of the othercomparator after the signal generated by said device expires.
 12. Theinvention of claim 4: said control means further including a deviceresponsive to the output of said first comparator for generating asignal for a predetermined time period, a window comparator forcomparing said first intermediate signal with a pair of referencesignals and generating an output signal if the first intermediate signalis greater than one of the reference signals but less than the otherreference signal, and gating means inhibiting the output of said firstcomparator unless said window comparator and said device produces anoutput signal.
 13. The invention of claim 10; and a warning devicemounted in the operator''s compartment of the vehicle; said controlmeans further including means for actuating said warning device if theoutput of the corresponding gating means is not equal to the output ofsaid one comparator.
 14. The invention of claim 11; and a warning devicemounted in the operator''s compartment of the vehicle; said controlmeans further including means for actuating said warning device if theoutput of said first gating means is not equal to the output of said onecomparator and if the output of said second gating means is not equal tothe output of the other comparator.
 15. The invention of Claim 4: awarning device mounted in the operator''s compartment of the vehicle;and first and second power amplifiers for amplifying said first andsecond error signals respectively; said control means further includingmeans for actuating said warning device when either of said first orsecond power amplifiers malfunction.
 16. In a tractor-trailer breakingsystem having a fluid pressure source and brake actuators operated byfluid pressure: a first fluid reservoir carried by said tractor; asecond fluid reservoir carried by said trailer; a relay valve carried bysaid trailer for actuating the brake actuators carried by the trailer,said relay valve having a primary section and a delivery section, saidprimary section being responsive to fluid pressure to cause saiddelivery section to communicate said second reservoir with the brakeactuators carried by the trailer; electrically operated valve means forcontrolling fluid communication between said primary section and saidsecond reservoir and between said primary section and the atmosphere; anoperator-operated brake control valve communicated to said firstreservoir for actuating the brakes of the tractor and trailer; controlmeans for comparing the fluid pressure level at the brake control valveand the fluid pressure level at the brake actuators carried by thetrailer and generating a first error signal if the pressure level at thebrake control valve exceeds the pressure level delivered to said traileractuators by more than a predetermined amount and generating a seconderror signal if the pressure level delivered to said trailer brakeactuators exceeds the pressure level at said control valve by more thana predetermined amount; said electrically operated valve means beingresponsive to said first error signal to communicate said primarysection of the relay valve with said second reservoir to thereby causesaid delivery section to communicate saiD second reservoir with thebrake actuators carried by the trailer and responsive to said seconderror signal to vent said primary section to atmosphere to therebypermit said delivery section to also vent said trailer brake actuatorsto atmosphere.
 17. The invention of claim 16: said brake control valvebeing communicated to said primary section to actuate said relay valvewhen a malfunction prevents normal operation of the electricallyoperated valve means; said electrically operated valve means terminatingfluid communication between the brake valve and the primary section whencommunication between said second reservoir and said primary section isinitiated.
 18. In a vehicle brake actuation system having a fluidpressure source and brake actuators operated by fluid pressure: anoperator-operated brake control valve communicated with said fluidpressure source; control means for comparing the difference between saidfluid pressure level at the brake control valve and at said actuatorswith a first reference value and generating a first error signal if saiddifference exceeds said reference value and terminating said first errorsignal when said difference drops below said predetermined amount, saidcontrol means comparing the difference between the fluid pressure levelat said control valve with a second reference value and generating asecond error signal if said last-mentioned difference exceeds saidsecond reference value and terminating said second error signal whensaid last-mentioned difference drops below said second reference value;and valve means responsive to said first error signal to communicatesaid actuators with said fluid pressure source and responsive to saidsecond error signal to vent said actuators.